1. Field of the Invention
The present invention relates to a planar inertial sensor, in particular for portable devices having a stand-by function, of micro-electro-mechanical-system (MEMS) type.
In particular, the subject matter of the present invention is an inertial sensor for detecting movement that is able to generate a capacitive variation signal, as a function of an input acceleration having any direction parallel to the plane of the sensor.
2. Description of the Related Art
Inertial devices for detecting movement are known and are able to detect acceleration in one or more directions. They generally comprise a moving element and a fixed body, the moving element being able to move with respect to the fixed body in response to an acceleration. The movement of the moving element brings about the variation of an electrical signal, which can be used by an appropriate control circuit.
Most known devices for detection of movement are currently formed by objects of large dimensions.
For example, inertial devices are known consisting of a glass bulb filled with mercury or other conductive liquids and accommodating a plurality of electrodes that enable detection of an electrical conductivity variation in the case of contact between the electrodes and the liquid following upon a displacement of the device.
Also known (see for example the U.S. Pat. No. 4,337,402) are switches for detecting movement, formed by a non-conductive cylindrical body accommodating a plurality of electrical contacts forming parallel tracks arranged along the circumference thereof, and a conductive bail, suspended in proximity of the electrical contacts via two conductive springs. When moving in a horizontal or vertical direction, the ball comes into contact with the conductive tracks, generating an activation signal.
More recently, the use of MEMS technology has been proposed for inertial movement-detection sensors. These sensors comprise a mobile part and a fixed part, both of conductive material (silicon and/or metal), capacitively coupled to one another. The movement of the mobile part with respect to the fixed one in presence of an acceleration brings about a capacitance variation detectable by an associated control circuit.
In particular, when the mobile part is formed by a suspended moving element, in case of an acceleration acting on the device, the suspended moving element moves with respect to the fixed part according to the translation, or rotation, degrees of freedom allowed to the suspended moving element according to the specific suspension type. The degrees of freedom correspond to the preferential axes of detection of the sensor in so far as the displacement of the moving element is maximum when the acceleration has a direction parallel to one of these axes and is considerably smaller otherwise.
Currently, inertial sensors perform detection of the movement in one or at the most two preferential directions of detection, for example ones parallel to the plane of the suspended moving element. When it is desired to detect the movement with a number of preferential directions of detection it is therefore necessary to replicate the sensor an appropriate number of times within the same device, with obvious problems of increase in dimensions and circuit complexity.
In the specific case of inertial switches, their operation envisages generation of a movement detection pulse as soon as the acceleration component in one of the preferential detection directions exceeds a certain threshold. Usually, an appropriate value is attributed to this threshold so as to prevent false detection due to disturbance of various nature; in general the detection threshold is the same for all the directions of movement.
The drawback of these inertial devices is due to the fact that, in case of movements in a plane direction different from the preferential detection axes, the acceleration necessary for exceeding the threshold is higher than in case of movements made in the preferential detection directions. Consequently, an acceleration, albeit having an intensity higher than the preset threshold, cannot be detected if its direction departs significantly from the preferential detection axes and its components along the preferential detection axes are smaller than the preset threshold.
In order to overcome this problem, commonly known as threshold error, it is possible to lower the detection threshold; this solution is not, however, satisfactory in so far as it causes the detection of disturbance even of modest intensity.
In U.S. patent application Ser. No. 10/788,962, filed on Feb. 27, 2004 in the name of the present applicant, the use is proposed of an inertial device comprising two movement sensors, each having a respective preferential detection direction (orthogonal to one another), and two detection thresholds having different values. In particular, a first threshold, of higher value, relates to accelerations directed in one of the preferential detection directions, and a second threshold, of lower value, relates to accelerations directed in different directions. Basically, the device detects a movement when at least one of the two components of the acceleration signal along the detection axes is greater than the first upper threshold, or else when both the components are simultaneously greater than the second lower threshold.
In this way, the threshold errors are sensibly reduced but the reading electronics must be doubled since it must comprise twice as many threshold comparators and an additional logics for controlling the logic relations between the signals at output from the comparators.
The same technique can moreover be extended so as to reduce further the threshold error by including a larger number of preferential detection directions and hence of threshold comparators and additional thresholds, with the obvious disadvantage, however, of increasing further the complexity of the reading electronics.